The present invention relates to liquid crystalline media comprising a first dielectrically positive component (component A) comprising one or more compounds of formula I, which have high values of xcex94n 
wherein
R1 is n-alkyl, n-alkoxy with 1 to 7 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms,
X1 is F, Cl, CF3, OCF3 or OCF2H and
one of Y11, Y12, Y13, Y14, Y15 and Y16 
is F and the others are independently of each other H or F, and
simultaneously
a second dielectrically positive component (component B) comprising one or more compounds of formula II 
wherein
R2 is n-alkyl, n-alkoxy with 1 to 7 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms,
X2 is F, Cl, CF3, OCF3 or OCF2H and
Y2 H or F,
to the use of these media especially in liquid crystal displays and to LCDs comprising these media, preferably to displays of the TN mode and in particular to displays addressed by an active matrix.
Liquid Crystal Displays (LCDs) are widely used to display information. They are used for direct view displays, as well as for projection type displays. Probably the most widely employed electro-optical mode still is the twisted nematic (TN)-mode. Besides this mode also other modes are already used or being investigated. Amongst the effects widely used are e.g. the super twisted nematic (STN)-effect, which is used for directly driven displays and the electrically controlled birefringence (ECB)-mode, also called vertically aligned nematic (VAN)-mode, which is used in active matrix addressed displays (AMDs), as well as their modifications. The TN-mode is beneficially used both in directly addressed displays and in AMDs.
A promising electrooptical mode for LCDs is the optically compensated bend (OCB) mode. This mode has a favorable small viewing angle dependence of the contrast. Further the response times in this mode are small.
Besides these modes, which all do use an electrical field, which is substantially perpendicular to the substrates, respectively to the liquid crystal layer, there are also electro-optical modes employing an electrical field substantially parallel to the substrates, respectively the liquid crystal layer like e.g. the in-plane switching (IPS)-mode (as disclosed e.g. in DE 40 00 451 and EP 0 588 568).
For TN-AMDs liquid crystalline media with an excellent resistivity are required. For direct view TN-AMDs with small cell gaps, as well as for reflective TN-AMDs with very small cell gaps, liquid crystalline media with higher birefringence (xcex94n) are required compared to commonly used direct view TN-AMDs with cell gaps of about 4 to 5 xcexcm, operating in the first transmission minimum according to Gooch and Tarry. Especially for modern TFT driven displays with polycrystalline silicon as the active medium such media with elevated xcex94n are required.
Liquid crystalline media containing compounds of formula I are known from DE 195 29 106. These media, however, are characterized by a low birefringence and have a rather high threshold voltage. Liquid crystalline media containing compounds of formula II with a terminal Cl atom are known from U.S. Pat. No. 5,328,644. These media, however, are characterized by a high birefringence.
Similar media are disclosed in JP 06-264 059 (A).
Thus, there is a significant need for liquid crystalline media with suitable properties for practical applications such as a wide nematic phase range, low viscosities, especially low rotational viscosities, appropriate optical anisotropy xcex94n according to the display mode used especially a suitably high xcex94n for TN-AMDs with small cell gaps.
Surprisingly, it now has been found that liquid crystal media with suitable xcex94n especially useful for TN-AMDs can be realized which do not exhibit the drawbacks of the materials of the prior art or at least do exhibit them to a significantly lesser degree. These improved liquid crystal media according to the instant invention are realized by using at least two components:
a first dielectrically positive liquid crystal component (called component A), comprising compounds of formula I, which are strongly dielectrically positive compounds with high values of xcex94n 
wherein
R1 is n-alkyl, n-alkoxy with 1 to 7 C-atoms, preferably 2 to 5 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms,
X1 is F, Cl, CF3, OCF3 or OCF2H, preferably F or Cl, most preferably F and
one of Y11, Y12, Y13, Y14, Y15 and Y16 
is F and the others are, independently of each other, H or F, preferably at least one of the others is F
and simultaneously a second dielectrically positive component (component B) comprising one or more compounds of formula II 
wherein
R2 is n-alkyl, n-alkoxy with 1 to 7 C-atoms, preferably 2 to 5 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms,
X2 is F, Cl, CF3, OCF3 or OCF2H, preferably F or Cl, most preferably F and
Y2 H or F, preferably F.
In a preferred embodiment, the liquid crystalline media according to the present invention comprise a further dielectrically positive component (called component C) which is comprising dielectrically positive compounds of formula III 
wherein
R31 and R32 independently of each other, have the meaning given for R1 under formula I above,
Z31 and Z32 are, independently of each other, xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, trans- CHxe2x95x90CHxe2x80x94, trans- xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CF2Oxe2x80x94 or a single bond, if both are present, preferably at least one of them is a single bond, 
xe2x80x83or their mirror images, 
X3 is F, Cl, halogenated alkyl, halogenated alkenyl or halogenated alkoxy, each having 1 to 6 C atoms, preferably F, Cl, OCF3 or OCF2H and
n is 0 or 1,
preferably
Z31 and Z32 Z are, independently of each other, xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, trans- CHxe2x95x90CHxe2x80x94, xe2x80x94CF2Oxe2x80x94 or a single bond, if both are present, preferably at least one of them is a single bond,
and/or preferably 
and/or preferably 
In yet a further preferred embodiment, the liquid crystalline media according to the present invention additionally comprise a dielectrically neutral component (called component D) which is comprising dielectrically neutral compounds of formula IV 
wherein
R41 and R42 independently of each other, have the meaning given for R1 under formula I above,
Z41 and Z42 are, independently of each other, xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, trans- CHxe2x95x90CHxe2x80x94, trans- xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CF2Oxe2x80x94 or a single bond, if both are present, preferably at least one of them is a single bond, 
xe2x80x83each have the meaning given for 
xe2x80x83above under formula I and
m is 0, 1 or 2.
Preferably the liquid crystalline media according to the instant invention contain a component A comprising, preferably predominantly consisting of and most preferably entirely consisting of, one or more compounds of formula I.
Comprising in this application means in the context of compositions, that the entity referred to, e.g. the medium or the component, contains the compound or compounds in question, preferably in a total concentration of 10% or more and most preferably of 20% or more.
Predominantly consisting, in this context, means that the entity referred to contains 80% or more, preferably 90% or more and most preferably 95% or more of the compound or compounds in question.
Entirely consisting, in this context, means that the entity referred to contains 98% or more, preferably 99% or more and most preferably 100.0% of the compound or compounds in question.
The compounds of formula I are preferably selected from the group of sub-formulae I-1 to I-3
wherein
R1 and X1 have the respective meanings given under formula I above.
Especially preferred are media comprising compounds selected from the group of sub-formulae I-1a, I-1b, I-2a and I-2b 
wherein
R1 has the meaning given under formula I above.
In a preferred embodiment the liquid crystalline media according to the instant invention contains a component B comprising, preferably predominantly consisting of and most preferably entirely consisting of, compounds of formula II.
The compounds of formula II are preferably selected from the group of sub-formulae II-1 to II-7
wherein
R2 and Y2 have the respective meanings given under formula II above.
Especially preferred are media comprising compounds selected from the group of sub-formulae II-1a, II-1b, II-6a and II-6b 
wherein
R2 has the meaning given under formula II above.
In a further preferred embodiment the liquid crystal medium contains a dielectric positive liquid crystal component (component C) which is preferably predominantly consisting of and most preferably entirely consisting of compounds of formula III as given above.
The compounds of formula III are preferably selected from the group of sub-formulae III-1 to III-12
wherein
R3 has the meaning given under formula III above,
Y31 and Y32 are independently of each other H or F and,
X3 has the meaning given under formula III above and is preferably F, OCF3 or OCF2H and
n is 0 or 1.
This component C may be present, and preferably is present, besides component B.
Especially preferred are media comprising compounds selected from the group of sub-formulae III-1a to III-1h and III-2 to III-10
wherein the parameters have the meaning given under formula III above.
In a further preferred embodiment the liquid crystal medium contains a dielectric positive liquid crystal component (component D) which is preferably predominantly consisting of and most preferably entirely consisting of compounds of formula IV as given above.
The compounds of formula IV are preferably selected from the group of sub-formulae IV-1 to IV-4
wherein
R41, R42 and m have the respective meanings given under formula IV above and Y4 is H or F.
Most preferably the medium contains compounds of formula I selected from the group of sub-formulae I-1 to I-3. Most prefered of these are compounds of sub-formulae I-1a, I-1b, I-2a, I-2b, I-3a and I-3b
wherein
R1 has the meaning given under formula I above and preferably is n-alkyl with 1 to 5 C-atoms or n-alkoxy with 1 to 4 C-atoms, or 1-E-alkenyl with 2 to 5 C-atoms.
Most prefered the media contain compounds selected from the group of formulae I-1a, I-1b and I-2a and particularly preferred from the group of formulae I-1a and I-2a.
Preferably the media contain compounds selected from the group of formulae III-4a to III-4d and III-5a to III-5c, preferably one or more compounds selected from the group of formulae III-4a to III-4d and one or more compounds selected from the group of formulae III-5a to III-5c 
wherein R3 has the meaning given under the formula III-4 above.
Component A is preferably used in a concentration of from 1 to 55%, more preferably from 3 to 40% and most preferably from 5 to 30% of the total mixture, by weight.
Component B is preferably used in a concentration from 1 to 45%, more preferably from 3 to 35% and most preferably from 5 to 25% of the total mixture by weight.
Component C is preferably used in a concentration from 0 to 85%, more preferably from 20 to 80% and most preferably from 40 to 75% of the total mixture.
Component D is preferably used in a concentration from 0 to 35%, more preferably from 1 to 25% and most preferably from 11 to 19% of the total mixture.
Optionally, the inventive media can comprise further liquid crystal compounds in order to adjust the physical properties. These further compounds are used to adjust especially the phase range, the optical anisotropy and the operating voltage of the inventive liquid crystal media. Such compounds are known to the expert. Their concentration in the media according to the instant invention is preferably 0% to 30%, more preferably 0% to 20% and most preferably 0% to 15%.
Preferably the liquid crystal medium contains 50% to 100%, more preferably 70% to 100% and most preferably 80% to 100% and in particular 90% to 100% totally of components A, B, C and D, which contain, preferably predominantly consist of and most preferably entirely consist of one or more of compounds of formulae I, II, III, and IV, respectively.
The liquid crystal media according to the instant invention are characterized by a clearing point above 80xc2x0 C., preferably of 90xc2x0 C. or more, especially preferred of 100xc2x0 C. or more and in particular of 110xc2x0 C. or more.
The xcex94n of the liquid crystal media according to the instant invention is 0.12 or more, preferably in the range of 0.13 to 0.25, more preferably in the range of 0.14 to 0.22, most preferably in the range of 0.14 to 0.20 and in particular in the range of 0.145 to 0.170.
The dielectrical anisotropy (xcex94xcex5) of the liquid crystalline media according to the invention, at 1 kHz and 20xc2x0 C., preferably is 6 or more, more preferably 9 or more, most preferably 10 or more and in particular 12 or more.
The threshold voltage (V10) of the liquid crystalline media according to the invention, at 50 Hz and 20xc2x0 C., preferably is 2.0 V or less, more preferably 1.7 V or less, most preferably 1.6 V or less and in particular 1.5 V or less. Preferably it is in the range from 1.2 to 1.6 V, most preferably in the range from 1.3to 1.5 V.
Preferably the nematic phase of the inventive media extends at least from xe2x88x9220xc2x0 C. to 70xc2x0 C., more preferably at least from xe2x88x9230xc2x0 C. to 80xc2x0 C. and most preferably at least from xe2x88x9240xc2x0 C. to 80xc2x0 C., wherein at least means that preferably the lower limit is under cut, wherein the upper limit is surpassed.
In the present application the term dielectrically positive compounds describes compounds with xcex94xcex5 greater than 1,5, dielectrically neutral compounds are compounds with xe2x88x921,5xe2x89xa6xcex94xcex5xe2x89xa61,5 and dielectrically negative compounds are compounds with xcex94xcex5 less than xe2x88x921,5. The same holds for components. xcex94xcex5 is determined at 1 kHz and 20xc2x0 C. The dielectrical anisotropies of the compounds is determined from the results of a solution of 10% of the individual compounds in a nematic host mixture. The capacities of these test mixtures are determined both in a cell with homeotropic and with homogeneous alignment. The cell gap of both types of cells is approximately 10 xcexcm. The voltage applied is a rectangular wave with a frequency of 1 kHz and a root mean square value typically of 0.5 V to 1.0 V, however, it is always selected to be below the capacitive threshold of the respective test mixture.
For dielectrically positive compounds the mixture ZLI-4792 and for dielectrically neutral, as well as for dielectrically negative compounds, the mixture ZLI-3086, both of Merck KGaA, Germany are used as host mixture, respectively. The dielectric permittivities of the compounds are determined from the change of the respective values of the host mixture upon addition of the compounds of interest and are extrapolated to a concentration of the compounds of interest of 100%.
Components having a nematic phase at the measurement temperature of 20xc2x0 C. are measured as such, all others are treated like compounds.
The term threshold voltage refers in the instant application to the optical threshold and is given for 10% relative contrast (V10) and the term saturation voltage refers to the optical saturation and is given for 90% relative contrast (V90) both, if not explicitly stated otherwise. The capacitive threshold voltage (V0, also called Freedericksz-threshold VFr) is only used if explicitly mentioned.
The ranges of parameters given in this application are all including the end points of the ranges, unless explicitly stated otherwise.
Throughout this application, unless explicitly stated otherwise, all concentrations are given in mass percent and relate to the respective complete mixture, all temperatures are given in degrees centigrade (Celsius) and all differences of temperatures in degrees centigrade. All physical properties have been and are determined according to xe2x80x9cMerck Liquid Crystals, Physical Properties of Liquid Crystalsxe2x80x9d, Status November 1997, Merck KGaA, Germany and are given for a temperature of 20xc2x0 C., unless explicitly stated otherwise. The optical anisotropy (xcex94n) is determined at a wavelength of 589.3 nm. The dielectric anisotropy (xcex94xcex5) is determined at a frequency of 1 kHz. The threshold voltages, as well as all other electro-optical properties have been determined with test cells prepared at Merck KGaA, Germany. The test cells for the determination of xcex94xcex5 had a cell gap of 22 xcexcm. The electrode was a circular ITO electrode with an area of 1.13 cm2 and a guard ring. The orientation layers were lecithin for homeotropic orientation (xcex5∥) and polyimide AL-1054 from Japan Synthetic Rubber for homogeneuous orientation (xcex5xe2x8axa5). The capacities were determined with a frequency response analyser Solatron 1260 using a sine wave with a voltage of 0.3 Vrms. The light used in the electro-optical measurements was white light. The set up used was a commercially available equipment of Otsuka, Japan. The characteristic voltages have been determined under perpendicular observation. The threshold (V10)xe2x80x94mid grey (V50)xe2x80x94and saturation (V90) voltages have been determined for 10%, 50% and 90% relative contrast, respectively.
The liquid crystal media according to the present invention can contain further additives and chiral dopants in usual concentrations. The total concentration of these further constituents is in the range of 0% to 10%, preferably 0.1% to 6%, based on the total mixture. The concentrations of the individual compounds used each are preferably in the range of 0.1 to 3%. The concentration of these and of similar additives is not taken into consideration for the values and ranges of the concentrations of the liquid crystal components and compounds of the liquid crystal media in this application.
The inventive liquid crystal media according to the present invention consist of several compounds, preferably of 6 to 40, more preferably of 8 to 30 and most preferably of 10 to 20 compounds. These compounds are mixed in conventional way. As a rule, the required amount of the compound used in the smaller amount is dissolved in the compound used in the greater amount. In case the temperature is above the clearing point of the compound used in the higher concentration, it is particularly easy to observe completion of the process of dissolution. It is, however, also possible to prepare the media by other conventional ways, e.g. using so called pre-mixtures, which can be e.g. homologous or eutectic mixtures of compounds or using so called multi-bottle-systems, the constituents of which are ready to use mixtures themselves.
By addition of suitable additives, the liquid crystal media according to the instant invention can be modified in such a way, that they are usable in all known types of liquid crystal displays, either using the liquid crystal media as such, like TN-, TN-AMD, ECB- or VAN-AMD IPS and OCB LCDs and also in composite systems, like PDLC-LCDs and especially in HPDLCs.
The melting point T(C,N), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T (N,I) of the liquid crystals are given in degrees centigrade.
In the present application and especially in the following examples, the structures of the liquid crystal compounds are represented by abbreviations also called acronyms. The transformation of the abbreviations into the corresponding structures is straight forward according to the following two tables A and B. All groups CnH2n+1 and CmH2m+1 are straight chain alkyl groups with n respectively m C-atoms. The interpretation of table B is self evident. Table A does only list the abbreviations for the cores of the structures. The individual compounds are denoted by the abbreviation of the core followed by a hyphen and a code specifying the substituents R1, R2, L2 and L2 follows:
The liquid crystal media according to the instant invention do contain preferably
nine or more, preferably eleven or more, compounds selected from the group of compounds of tables A and B and/or
four or more, preferably five or more, compounds selected from the group of compounds of table A and/or
five or more, preferably eight or more, compounds selected from the group of compounds of table A.
The entire disclosure[s] of all applications, patents and publications, cited above or below, and of corresponding Application No. EP 01101238.2, filed Jan. 19, 2001, are/is hereby incorporated by reference.